I am a terrible driver and can have mishaps at the drop of a hat. For the scenario you have described, I would consult the navigator and attempt to infer the route that has the most traffic. This route would be preferable to me because with lots of traffic one can get 'inside' a group of cars somewhere near the end. This would be my strategy if the road conditions are poor, uphill/downhill makes no difference.
Trucks and lorries that careen out of control would most likely take out those at the front of the group. And for visibility, you are better off with a string of tail lights in front of you so you can minimally stay on the road and be alerted to something when the 'group leaders' apply the brakes.
Most importantly, when something bad happens, there will be other drivers who (in Europe) are required to stop and give aid.
For you're asking for 'actual accident data', I can contribute some personal experiences. I was run off the road three times on the continent, one of them dangerously so.
The road to Mount Athos is basically flat until you reach the last city, Ouranopouli, and then it is full on treacherous. There are just narrow and twisty dirt roads with lots of 'thrilling' hairpins. In my case I was driving uphill and got run off by a lorry going downhill who was coming out of a hairpin. This was about 2AM, rainy, foggy, and all-around horrible conditions. The problem with Mount Athos is you don't get the choice of roads, the border has one drivable road leading to it and then you have to park and hike in. Drive in the day time. Optimally get behind a big lorry and follow them in/out until you're past the nature park. There's a good reason people prefer to take the ferry!
Another time on the Route Napoleon I got run off voluntarily by a group of motorcyclists who were passing me on a hairpin. Essentially I got spooked and pulled over to avoid any sort of accident caused by on coming traffic. I am an erstwhile guide on the Route Napoleon, but will never undertake it between September and May. In the summer there are lots of motorcycle clubs who converge on the route because of the 'thrills' it offers: no guard rails, mountainous roads, horrifying stone arches at the head of hairpin curves, and gorgeous scenery to die for. Again, leave Cannes early on a clear day, like 9 or 10AM and plan to arrive in Grenoble that afternoon or stop along the way well before dusk.
And another time on a twisty road outside of Bayreuth. Again this was voluntary because there wasn't enough room for me and the lorry approaching. No collision and no permanent damage, but jarring nonetheless.
See also: Britain's most dangerous roads (I will not even think about driving on these).
So to square the circle, the answer to your question which direction is safer, uphill or downhill; my answer is they are both equally safe/dangerous.